A career development plan is described for Dr. Nathaniel D. Collins in this Mentored Clinical Scientist Development Award (MCSDA) application. This plan is to provide Dr. Collins with the necessary skills to investigate molecular virology and pathogenesis with the ultimate goal of becoming an independent investigator. Dr. Collins is a veterinarian who is currently pursuing a Ph.D. degree in the Department of Veterinary Biosciences at The Ohio State University. He is co-sponsored by Dr. Michael Lairmore and Dr. Gary Kociba. The career development plan is divided into two phases. Phase I will last three years and consist of didactic training and practical research training conducted in the form of a research project. Phase II will last one year, will be a continuation of research initiated in phase I, and be conduced as post-doctoral studies with an emphasis on transition to independence as an investigator. Human T-lymphotropic virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia and HTLV-1 myelopathy. HTLV-1 infection is characterized by long periods of clinical latency. Information concerning pathogenesis and mechanisms of infectivity and latency is limited due, in part, to the past unavailability of an infectious molecular clone of HTLV-1. Using primary lymphocytes and the rabbit, which is a model for the asymptomatic carrier state seen in the majority of HTLV-1-infected individuals, Dr. Collins demonstrated a molecular clone of HTLV-1, designated ACH, to be infectious in vitro and in vivo as well as capable of transforming CD4+ T-lymphocytes, similar to wild- type HTLV-1. Using viral mutants derived from ACH, it is proposed to investigate the role of alternatively spliced genes in HTLV-1 infection. A region of the HVLV-1 genome, known as the pX region, contains four open reading frames which encode a number of proteins. Two of these proteins derived from open reading frames (ORFs) III and IV, tax and rex, are well-characterized viral regulatory proteins. Much less is known about proteins derived from ORFs I and II. Because of their expressed and conserved nature in HTLV-1-infected individuals, it is likely that they play a role in the HTLV-1 life cycle. In addition, homologous genes in HTLV-1 related viruses, BLV and HTLV-II, have been shown to be important for maintaining high viral loads in vivo. Preliminary results suggest that ACH-derived clones of HTLV-1 that are mutated to abrogate ORF I or II gene expression are replication competent. During specific aim 1, investigations will be conduced into the ability of these mutants cause proliferation, immortalization, and transformation in primary cells. Characterization of viral protein and mRNA production as well as cellular IL-2 receptor expression will be conducted to determine if these gene products influence viral or cellular gene regulation. Specific aim 2 research will focus on the in vivo infectivity of the mutants. Serology, viral isolation, quantitative PCR, and in situ PCR and hybridization will be used to detect differences in viral infectivity, replication, distribution, and expression.